As a liquid crystal display device, there is currently known a TN (Twisted Nematic) liquid crystal display device having a so-called TN liquid crystal cell which is obtained by forming a liquid crystal alignment film from an organic resin on the surface of a substrate having a transparent conductive film to produce a substrate for a liquid crystal display device, opposing two of the substrates to each other, forming a nematic liquid crystal layer having positive dielectric anisotropy in the gap between the two substrates to fabricate a sandwich-structured cell and twisting the long axis of each liquid crystal molecule at 90° continuously from one substrate to the other. Further, an STN (Super Twisted Nematic) liquid crystal display device which can achieve a higher contrast ratio than that of the TN liquid crystal display device, an IPS (In-Plane Switching) liquid crystal display device which has little dependence upon the view angle and a VA (Vertical Alignment) liquid crystal display device which comprises nematic liquid crystals having negative dielectric anisotropy have been developed.
The operation principles of these liquid crystal display devices are roughly divided into a transmission type and a reflection type. The transmission type liquid crystal display device displays, making use of a change in the intensity of light from a backlight source transmitted from the rear of the device at the time of driving the device. The reflection type liquid crystal display device displays, making use of a change in the intensity of reflected light from the outside such as sunlight at the time of driving the device without using a backlight source. Therefore, it is considered that this reflection type liquid crystal display device is especially advantageous when it is used outdoors as its power consumption is lower than that of the transmission type liquid crystal display device.
In the transmission type liquid crystal display device, a liquid crystal alignment film provided in the device is exposed to light from the backlight source for a long time. Especially in the application of the transmission type liquid crystal display device in a liquid crystal projector which is now in growing demand as a home theater in addition to its business application, a light source having very high irradiation intensity such as a metal halide lamp is used. It is also conceivable that the temperature of the liquid crystal display device itself rises at the time of driving due to the application of light having high intensity.
It is most likely that the reflection type liquid crystal display device is used outdoors. In this case, sunlight including strong ultraviolet radiation is a light source. The distance of light passing through the reflection type liquid crystal display device is longer than that of the transmission type liquid crystal display device theoretically.
Further, there is a trend toward the use of both the transmission type liquid crystal display device and the reflection type liquid crystal display device in private vehicles, and the use and installation environment of these display devices at a high temperature is becoming realistic as compared with the conventional use and installation environment of a liquid crystal display device.
In the production process of the liquid crystal display device, a liquid crystal dropping process, that is, ODF (One Drop Fill) process has begun to be used to shorten the process and improve the yield. In the ODF process, unlike the prior art process in which liquid crystals are injected into an empty liquid crystal cell assembled by using a thermosetting sealing agent, an ultraviolet curable sealing agent is applied to a required position of one substrate on which a liquid crystal alignment film has been formed, liquid crystals are dropped on required positions, the other substrate is joined to the above substrate, and ultraviolet radiation is applied to the entire surface of the obtained assembly to cure the sealing agent so as to fabricate a liquid crystal cell. The ultraviolet radiation to be applied at this point is usually strong with several Joule or more per cm2. That is, in the production process of the liquid crystal display device, the liquid crystal alignment film is exposed to this strong ultraviolet radiation together with the liquid crystals.
Thus, the liquid crystal display device is exposed to a harsh environment which has been inconceivable in the prior art, such as the application of high-intensity light, a high-temperature environment and long-term drive as it is more functional and used for more purposes and its production process is improved more. Even under the above environment, higher liquid crystal alignability, electric properties such as voltage holding ratio and higher display properties than those of the prior art are required and further longer service life is required for the liquid crystal display device.
As the material of the liquid crystal alignment film constituting the liquid crystal display device, there have been known organic resins such as polyimides, polyamic acids, polyamides and polyesters. In organic resins, especially polyimides have been used in many liquid crystal display devices because they have excellent physical properties such as heat resistance, affinity for liquid crystals and mechanical strength (refer to JP-A 9-197411, JP-A 2003-149648 and JP-A 2003-107486).
However, new requirements are becoming stronger for the liquid crystal display devices of these days due to the above harsh production environment and use environment, whereby the conventionally accepted heat resistance and light resistance which can be achieved by organic resins have become unsatisfactory.
Then, a liquid crystal alignment film having excellent heat resistance and light resistance is now under study. For example, JP-A 9-281502 discloses a homeotropic alignment type liquid crystal alignment film which is formed from a polysiloxane solution obtained from a silicon compound having 4 alkoxyl groups and a silicon compound having 3 alkoxyl groups and teaches that the film is excellent in homeotropic alignability, heat resistance and homogeneity and that the coating solution has high stability.
However, as the liquid crystal alignment film formed by the technology of JP-A 9-281502 does not satisfy the requirements from the current harsh production environment and use environment and the storage stability of the coating solution is unsatisfactory, it has a problem with convenience when it is industrially used.
A liquid crystal aligning agent which can provide a liquid crystal alignment film having sufficiently high heat resistance and light resistance even in the current very harsh production environment and use environment and which has excellent storage stability is not known yet.